This invention relates to a means to initiate and modify surface oxide layers in the form of metal and metal oxide layers with and without catalytic activity. The invention involves the mixing of structural metal powders with base metals, the selective mobilization of certain metals to the porous surface of the substrate body during firing, and partial to complete oxidation of the metals to form surface oxide layers.
Commonly, base metal oxides of the first and second transition series have been mixed as a unitary composition and subsequently sintered to a dense body. The resultant structure is then placed in a chemically charged environment to catalytically promote certain reaction products. It is additionally known, that the base metal oxides can be combined with a washcoat of some kind and applied to a support structure for subsequent use in a reactive environment.
The reactive environments contemplated by the present invention relate, but are by no means limited thereby, to catalytic devices and filters with high surface area catalytic reaction sites. For example, the problem of acid rain is becoming increasingly invidious to the environment. Extant is a critical need for a long term, stable device capable of catalytic activity yet resilient in the harsh environment inherent to polluting activities. Such an environment invites the use of the present invention in that the high surface areas are comprised of thin porous metal oxide layers, intimately combined with a porous metal support structure. The support structure is capable of providing unoxidized precursor metal catalysts which can be readily oxidized to form fresh catalytic activity when exposed to an oxidative environment.
Some of the problems existing in the prior art are: in substrates with coated surfaces the coating is subject to delamination thereby requiring device replacement, there is no means available to integrate a support material with a catalyst material without the problems associated with abrupt material discontinuities, there are limited means available to selectively migrate to the surface of a porous support structure certain metals, there are limited means available to engineer the surface and/or oxide layer, and there is no integration of the porous surface layers of mixed catalytically active metal oxides in combination with the precursor metal catalyst and support structure.
The present invention addresses and solves these problems of the prior art by: combining metal powders and metal catalysts, sintering to form a hard porous body with certain metals migrating to the surface of the body; formation of integral layers of metals, controlling the surface oxide composition by varying the bulk metal composition, oxidizing a certain portion of the surface to form mixed oxide layers on the surface portion; while retaining either singly or a plurality of underlying, integrally mixed layers of metal stock capable of transformation to a fresh oxide layer when exposed to an oxidative environment. The present invention provides the dynamic surface layers herebefore mentioned, yet additionally intimately combines the dynamic surface layers with a stable support structure. Such structures may be employed for many useful purposes as is disclosed hereinbelow.